In an air-conditioning apparatus of the related art that uses a fluorocarbon refrigerant, refrigerating machine oil that is readily soluble in a liquid refrigerant having miscibility therewith is used. This is particularly typical when used for the purpose of air conditioning a building, in which the refrigeration cycle is large-scaled and complicated, because it is advantageous in that the refrigerating machine oil is less likely to accumulate in areas where the liquid refrigerant exists within the cycle.
However, with a recent trend towards the use of natural refrigerants, such as carbon dioxide, mechanical components, such as a compressor, require refrigerating machine oil with high viscosity. High-viscosity refrigerating machine oil has low solubility in the refrigerant and thus tends to accumulate in the refrigeration cycle. If the refrigerating machine oil accumulates in a heat exchanger, the heat transfer performance would be reduced, and if the refrigerating machine oil accumulates in the heat exchanger, a piping, or a container, the amount of oil inside the compressor would be reduced. This would hinder reliability.
As a measure of the above, there has been proposed an apparatus in which an oil-return operation is performed when the compressor is operating at a low frequency by making an indoor heat exchanger function as a radiator and by increasing the frequency of the compressor (for example, see Patent Literature 1).